1. Field of the Invention
The present invention relates to a liquid crystal display device which uses a liquid crystal having a spontaneous polarization, a liquid crystal display apparatus and a liquid crystal driving method.
2. Description of the Related Art
Conventional liquid crystal display (LCD) devices generally use a nematic liquid crystal. But, LCD devices using a nematic phase suffer a narrow view angle (a narrow area from which a display image having a sufficient contrast can be observed).
Attention has recently been given to LCD devices which use a liquid crystal having a spontaneous polarization, such as a ferroelectric liquid crystal or an antiferroelectric liquid crystal. Such LCD device can display high-quality images and have a high response characteristic.
Studies to make a practical use of LCD devices using this kind of liquid crystal have been made mainly on a ferroelectric liquid crystal called "SS-F liquid crystal." The SS-F liquid crystal has a bistable (memory property) alignment state and is suitable for simple matrix driving. The SS-F liquid crystal however has such a shortcoming that the transmittance cannot be changed stepwisely, disabling gradation display.
As a solution to this problem, it has been proposed to effect the active matrix driving of the SS-F liquid crystal to steadily apply an intermediate voltage to the SS-F liquid crystal and to control the ratio of the white display area to the black display area to ensure pseudo gradation display.
According to this driving method, the aligning state of the liquid crystal changes even if the same voltage is applied to the liquid crystal. Therefore, it is difficult to display an arbitrary gradation stably and certainly.
To overcome this problem, ferroelectric liquid crystal LCD devices which can ensure stable gradation display have been studied, and there has been proposed ferroelectric liquid crystals whose chiral smectic phases have a helical pitch smaller than the gap between the substrates of the LCD device. Of the ferroelectric liquid crystals of this type, one which has a memory, property is called an SBF liquid crystal and one which has a non-memory property is called a DHF liquid crystal (see "LIQUID CRYSTALS," 1989, Vol. 5, NO. 4, pp 1171 to 1177).
When a voltage whose absolute value is sufficiently large is applied between the opposing electrodes which sandwich the liquid crystal (LC) layer, the DHF liquid crystal becomes either a first alignment state where the directors of the LC molecules are substantially aligned in a first direction or a second alignment state where the directors of the LC molecules are substantially aligned in a second direction in accordance with the polarity of the applied voltage. When the absolute value of the applied voltage is smaller than the one which sets the first alignment state or the second alignment state, the DHF liquid crystal becomes an intermediate alignment state where the average direction of the directors of the LC molecules comes between the first and second directions due to the deformation of helical structure which the molecules draw, thus ensuring intermediate gradation display.
When a voltage whose absolute value is equal to or greater than a predetermined value is applied between the opposing electrodes which sandwich the LC layer, the SBF liquid crystal becomes either a first alignment state where the average of the directions of lengthwise axes, i.e., director of the LC molecules are substantially aligned in a first direction or a second alignment state where the directors of the LC molecules are substantially aligned in a second direction in accordance with the polarity of the applied voltage. When the absolute value of the applied voltage is smaller than the one which sets the first alignment state or the second alignment state, the SBF liquid crystal becomes an intermediate alignment state where the LC molecules whose directors are aligned in the first direction are mixed with the LC molecules whose directors are aligned in the second direction, thus ensuring intermediate gradation display.
Therefore, intermediate gradation can stably and certainly be displayed by keeping applying the intermediate voltage to the DHF liquid crystal or the SBF liquid crystal by active driving during the non-selection period.
The DHF liquid crystal, the SBF liquid crystal and the like have a spontaneous polarization. Therefore, the LCD device of this type has a larger capacitance of each pixel which is formed by a pixel electrode, the common electrode and the liquid crystal material interposed therebetween than that of the liquid crystal device using the nematic liquid crystal. Thus, the LCD device using the liquid crystal having the spontaneous polarization takes time to charge the individual pixels (to store charges), so that while this LCD device is characterized by the fast behavior of the LC molecules due to the interaction of the spontaneous polarization and the electric field, it suffers the long writing time of from a pixels and slow operation.
When the writing time (selection time) of the individual pixels is long, the number of displayable pixels is reduced so that a high image quality cannot be maintained. If the number of rows of pixels is kept constant, the frame frequency gets lower and displayed images suffer apparent flickering, which may result in the frame loss of moving pictures.
A simple way to shorten the writing time of the pixels is to set the voltage to be applied to the liquid crystal higher. The higher applied voltage, however, means that the breakdown voltages of the LCD device and the driving circuit should be increased.